Return path is defined in the RFC for email. As far as I am aware only the recipient’s mail server is supposed to add a Return-Path header. SO over riding it is Thunderbird should not be possible as it should not set it.

Just be clear, what happens with google in SMTP is nothing like what happens in Google with web mail. Using one as a reason for anything just does not fly for gmail.

Perhaps you would be best served to send your mail using the domains the accounts are associated with. Using GMail to cover all outgoing mail sort of works, but the recipient servers might tend to classify mail from gmail with other domains as spam. Or as you have identified, record where they are coming from.

Thanks Matt,

If I understood you correctly, it's actually the Gmail MX server appending the Return-Path header per SMTP protocol, while the web portal is in fact not using SMTP thus no such header is appended through web mail.

Gmail Help sort of confirms this: Important: While these directions let you send emails from a custom email alias at your domain, email recipients can still find your personal Gmail address if they inspect the email headers.

Since each user is assigned a single Login / SMTP credential per Primary Domain name (and no additional Login / SMTP credentials exist for the Alias Domains), the best solution is to set the Primary Domain to the Business DN, because I don't mind if the Business DN is revealed in headers. This seems like the most professional looking solution.

Thanks.